Autotaxin, also known as ATX, ENPP2 or NPP2, short for Ectonucleotide pyrophosphatase phosphodiesterase 2 is an enzyme secreted within the human body. This molecule has been known for generating (LPA) through conversion of lysophosphatidyl-choline (LPC) thereto via lysophospholipase D activity (the removal of choline from the base compound generates LPA). LPA has been realized to contribute to tumor cell growth, unfortunately, as the reactivity within the human body of LPA within certain tissues has resulted, in certain studies, of cancerous growths when present at certain levels. In this manner, then, it has been theorized that the greater the incidence of autotaxin activity within the human body, the greater the possibility of LPA generation. A reduction in the catalytic capabilities of autotaxin to convert the LPC molecule to LPA would theoretically permit an ultimate reduction in possibility of unwanted cell proliferation through reduced LPA presence within a subject's body.
The mechanism of autotaxin in terms of enzymatic activity and catalysis to form LPA resides in its phosphodiesterase capability. LPA can be generated from the cleavage of the phophodiester bonds of LPC, as well as its function as a phospholipase enzyme (note Formula I).
In extracellular fluids, this enzymatic catalysis of LPC removes the choline group, leaving LPA, which has a tendency to stimulate cell growth and proliferation as well as chemotaxis. From this, it appears that the motility of tumor cells is increased as well, resulting in properties and gene expression within certain carcinomas (such as, for instance, breast cancer cells), causing further processing into a form that is bioactive and potentially dangerous. Metastasis and oncogenesis of cancer cells appear to occur as well with elevated levels of LPA present within a targeted region. Increased ATX expression has been identified in renal carcinoma, metastatic breast cancer, thyroid carcinoma, Hodgkin lymphoma, and invasive glioblastoma multiforme.
It has thus been determined that the ability to prevent, or at least reduce, the amount of LPA within human body holds great promise at, likewise, reducing, if not preventing, the onset of certain cancers. It has been theorized, as noted above, that autotaxin modifications may prevent the undesirable conversion from LPC to LPA; the ability to actually accomplish such a result has been elusive, however, at least to the degree necessary for effective broad-scale utilization of such a method. Any modification thereof must exhibit an ability to drastically reduce the activity of autotaxin while also, preferably exhibiting oral bioavailability as well.
Past work at ATX inhibition has included L-histidine. Unfortunately, millimolar concentrations were required for any efficacy, and, more importantly, zinc sulfate presence (in submillimolar concentrations) suggested an inhibition mechanism involving interaction with the two native active site metal ions thereof. Other potential ATX inhibitors have included the products of ATX-catalyzed hydrolysis of LPC and sphingosyl phosphorylcholine (SPC), LPA, and S1P, respectively. Inhibition of ATX by LPA and S1P suggests that product feedback inhibition may contribute to regulation of ATX function in vivo. Previously reported ATX inhibitors share several common structural features, including a phosphate, thiophosphate, or phosphonate headgroup attached either with or without a linker to an alkyl chain, which can vary in overall length and can be either saturated or unsaturated. However, these compounds both lack substantial structural diversity and fail to meet Lipinski's empirical rules that characterize 90% of orally bioavailable drugs. It is of great importance to identify novel non-lipid structural classes capable of inhibiting ATX and which are orally bioavailable to treat certain tumor classes.
It is believed, without relying upon any specific scientific basis, that the lack of diversity in reported ATX inhibitors, as noted above, is due, in part, to the lack of a characterized three-dimensional structure of the enzyme. The ATX sequence of over 860 amino acids is divided into several domains, including a central catalytic domain composed of about 400 amino acids. ATX is a member of the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, as well as the alkaline phosphatase superfamily. Crystallographic structures of several alkaline phosphatase superfamily members have been available for decades. These crystal structures show remarkable structural conservation in a small core surrounding the catalytic site, but unfortunately show completely different structural characteristics outside this conserved core. Sequence homology of the alkaline phosphatases with ATX does not exceed 14% and is therefore insufficient for generation of a high quality homology model in any region outside the approximately 100 amino acid structurally conserved core. The recent report of a crystal structure of a bacterial NPP enzyme with 30% identity to the ATX catalytic core domain enabled the development of a structural model of the ATX catalytic domain that may prove useful in structure-based drug design. Although a significant improvement, such a homology model must be applied cautiously as involvement of the c-terminal nuclease-like domain in substrate recognition has been suggested from studies of NPP family domain-swapping chimeras. In any event, these previously reported ATX inhibitors are analogs of LPA, a phospholipid, and are more hydrophobic than is typical of orally bioavailable drugs, thereby creating problems in that area.
As such, there exists a definitive lack in providing effective ATX inhibition (or inactivation) within the current knowledge base in this area, particularly as it concerns compounds that not only exhibit ATX inhibition, but also meet certain oral bioavailability requirements (as measured by Lipinski's rules). No such need has been met until now.